append(Cons(a,x),y) = Cons(idNat(a),append(x,y))
append(Nil      ,y) = idList(y)

idList(Cons(a,x)) = Cons(idNat(a),idList(x))
idList(Nil)       = Nil

idNat(S(x)) = S(idNat(x))
idNat(Z)    = Z
